A limited cellular lifespan is hypothesized to play a crucial role in organismal aging. Despite the putative limited importance of cellular lifespan in aging, we currently have only a rudimentary understanding of the molecular process. Therefore, elucidation of the molecular mechanisms that control cellular lifespan will have far-reaching implications in the fields of aging, cancer biology, and tissue regeneration. When grown in culture, normal human cells divide a limited number of times before entering a state of replicative senescence, in which they remain viable but are unable to divide further. It is postulated that this limited replicative capacity contributes to the phenotypes associated with aging, such as reduced wound healing and a weakened immune system. Our interest in cellular lifespan is focused on the telomere - a specialized structure composed of DNA and proteins, which is located at the end of linear chromosomes - because it plays a key role in controlling cellular lifespan. This proposal will take a novel proteomics approach to identify telomeric binding proteins. While the identification of these proteins will be of great importance, delineation of their function in telomere homeostasis will be paramount to furthering our understanding of cellular aging. We will use these newly identified proteins, and proteins already known to bind the telomere, to set up a number of suppressor screens in human cells. Our expertise in vector-based RNAi systems makes these studies feasible. These latter studies will allow us to not only identify telomere binding proteins, but will also allow us to uncover critical components of the signal transduction pathways that impinge on the telomere by affecting its homeostasis and cellular lifespan. Currently we have several cell-based systems with which we can address the function of new proteins in telomere homeostasis and cellular lifespan. Our cell-based systems combined with the novel protein discovery systems we have proposed will prove to be a powerful arsenal with which we will begin to understand cellular aging. Understanding how telomere structure is monitored, maintained, and degraded will allow us to delineate the molecular mechanisms of senescence. Once we have an understanding of this process, we will be able determine whether senescence plays a causative role in organismal aging. For these reasons, a detailed understanding of the molecular mechanisms governing cellular senescence is critical.